A measuring device as used for measuring structures on substrates (wafers or masks) has been described in detail in the paper entitled “Pattern Placement Metrology for Mask Making” held by Dr. Carola Blasing at the Semicon Education Program Convention in Geneva on Mar. 31, 1998. The description given there is the basis of a coordinate measuring device. Since the present invention can be advantageously used with such a measuring device and will be primarily described with reference to such a measuring device, without prejudice to its general applicability, this measuring device will be described in the following with reference to annexed FIG. 1. The well-known measuring device 1 is for measuring structures 31 and their coordinates on a sample 30, such as masks and wafers. In the context of the present application, the terms “sample”, “substrate” and the general term “object” are to be regarded as synonymous. In the production of semiconductor chips arranged on wafers with ever increasing integration the structural widths of the individual structures 31 become ever smaller. As a consequence the requirements as to the specification of coordinate measuring devices used as measuring and inspection systems for measuring the edges and the positions of structures 31 and for measuring structural widths and overlay data become ever more stringent. Optical sampling techniques in the form similar to a microscope and in combination with a laser distance measuring system are still favored. The advantage of optical measuring devices is that they are substantially less complicated in structure and easier to operate when compared to systems with different sampling, such as X-ray or electron beam sampling, and their greater stability with respect to the position measurement.
The actual measuring system in this measuring device 1 is arranged on a vibration-damped granite block 23. The masks or wafers are placed on a measuring stage 26 by an automatic handling system. This measuring stage 26 is supported on the surface of granite block 23 by air bearings 27, 28. Measuring stage 26 is motor driven and displaceable in two dimensions (X and Y coordinate directions). The corresponding driving elements are not shown. Planar mirrors 9 are mounted on two mutually vertical sides of measuring stage 26. A laser interferometer system 29 comprising a plurality of interferometers is used to track and determine the position of measuring stage 26.
The illumination and imaging of the structures to be measured is carried out by a high-resolution microscope optics with incident light and/or transmitted light in the spectral range of the near UV (without prejudice to its general applicability). A CCD camera serves as a detector. Measuring signals are obtained from the pixels of the CCD detector array positioned within a measuring window. An intensity profile of the measured structure is derived therefrom by means of image processing, for example, for determining the edge position of the structure or the intersection point of two structures intersecting each other. Usually the positions of such structural elements are determined relative to a reference point on the substrate (mask or wafer) or relative to optical axis 20. Together with the interferometrically measured position of measuring stage 26 this results in the coordinates of structure 31. The structures on the wafers or masks used for exposure only allow extremely small tolerances. Thus, to inspect these structures, extremely high measuring accuracies (currently in the order of nanometers) are required. A method and a measuring device for determining the position of such structures is known from German Patent Application Publication DE 100 47 211 A1 and related U.S. Pat. No. 6,920,249, which is hereby incorporated by reference herein. For details of the above position determination explicit reference is made to these documents.
In the example of a measuring device 1 illustrated in FIG. 1, measuring stage 26 is formed as a frame so that sample 30 can also be illuminated with transmitted light from below. Above sample 30 is the incident-light, illumination and imaging device 2, which is arranged about an optical axis 20. (Auto)focusing is possible along optical axis 20 in the Z coordinate direction. Illumination and imaging means 2 comprises a beam splitting module 32, the above detector 34, an alignment means 33, and a plurality of illumination devices 35 (such as for the autofocus, an overview illumination, and the actual sample illumination). The objective displaceable in the Z coordinate direction is indicated at 21.
A transmitted-light illumination means with a height adjustable condenser 17 and a light source 7 is also inserted in granite block 23, having its light received via an enlarged coupling-in optics 3 with a numerical intake aperture which is as large as possible. In this way as much light as possible is received from light source 7. The light thus received is coupled-in in the coupling-in optics 3 into a light guide 4 such as a fiber-optic bundle. A coupling-out optics 5 which is preferably formed as an achromatic objective couples the light into condenser 17. The illumination light can also be coupled-in from light source 7 via a mirror assembly.
In order to achieve the required nanometer accuracy of the structural measurement it is essential to minimize as far as possible interfering influences from the environment, such as changes in the ambient air or vibrations. For this purpose the measuring device can be accommodated in a climate chamber which controls the temperature and humidity in the chamber with great accuracy (<0.01° C. or <1% relative humidity). To eliminate vibrations, as mentioned above, measuring device 1 is supported on a granite block with vibration dampers 24, 25.
The accuracy of determining the position of the structures is highly dependent on the stability and accuracy of the laser interferometer systems used for determining the stage position in X and Y coordinate directions. Since the laser beams of the interferometer propagate in the ambient air of the measuring device, the wavelength depends on the refractive index of this ambient air. This refractive index changes with changes in the temperature, humidity and air pressure. Despite the control of temperature and humidity in the climate chamber, the remaining variations of the wavelength are too strong for the required measuring accuracy. A so-called etalon is therefore used to compensate for measuring value changes due to changes in the refractive index of the ambient air. In such an etalon a measuring beam covers a fixed metric distance so that changes in the corresponding measured optical length can only be caused by changes in the measuring index of the ambient air. This is how the influence of a change in the refractive index can be largely compensated with the etalon measurement by continuously determining the current value of the wavelength and taking it into account for the interferometric measurement.
To achieve the highest accuracy, the laser distance measuring system is usually operated according to the heterodyne principle, which uses the possibility of splitting the laser beam into the two linearly polarized components (herein, the small frequency difference of the two Zeemann lines is used). The two components are split up in the interferometer, are used as measuring and reference beams, and again superimposed in the interferometer and made to interfere with each other. The laser distance measuring system used has a resolution of 0.309 nm per integer value (laser click) of the laser distance measuring system, at a wavelength λLaser of the Laser of 632.8 nm.
To describe the accuracy of the measuring device described, usually the threefold standard deviation (3σ) of the measured average value of a coordinate is used. In a normal distribution of measuring values, statistically 99% of the measuring values are within a 3σ range about the average value. Indications as to repeatability are made by measuring a grid of points in the X and Y coordinate directions, wherein for each direction, after repeated measuring of all points, an average and a maximum 3σ value can be indicated. As a typical example, crossed chromium structures having a width of 1 μm of a 15×15 grid (pitch of the grid points: 10 mm) are measured on a quartz substrate. The measurement 2× (X and Y) 225points is repeated 20 times (20 passes). After a so-called multipoint correction, which allows all points of a pass to be commonly rotated and shifted, a repeatability (maximum value 3σ of the deviation of all 3σ values of the 450 points) of 1.5 to 2 nm is achieved. Without the multipoint correction, the values are between 2 and 6 nm.
A further improvement of the repeatability and therefore of the measuring accuracy of the measuring device described is desirable. Special attention has been paid in the present invention to the laser interferometer used for coordinate measurement of the measuring stage or for determining changes in the coordinates of this measuring stage. It is noted that the present invention is not limited to interferometers in the context of the measuring device described but can generally be used in laser-interferometric measurements.
From U.S. Pat. No. 5,469,260 an apparatus is known for measuring the position of a one or two dimensionally traversable stage by means of laser interferometry. For this purpose a stationary mirror is attached, for example, on the stationary optical system while the moveable stage carries a mirror along with it. In the well-known manner a laser beam is split in such a way that one part is incident on the stationary mirror while the other part is incident on the mirror which is carried along, and reflected on it. The reflected partial beams are made to interfere with each other wherein, by displacing the interference rings, a relative displacement of the mirror carried along with respect to the stationary mirror can be derived and the amount of this displacement can be determined.
As an example of the above measuring system, the position measurement of a wafer support stage during exposure of a wafer through a mask and an optical projection system (stepper) are discussed in the present document. Herein the position of the support stage relative to the stationary optical projection system is measured by means of interferometry. To measure the X and Y coordinates of the stage in a plane two interferometer systems are therefore necessary.